Reverse Spillover of Hydrogen on Carbon-Based Nanomaterials: Evidence of Recombination Using Isotopic Exchange

The hydrogen spillover phenomenon at ambient temperature is of interest in the development of adsorbents for storage applications. The mechanism of hydrogen spillover is studied using equilibrium dosing of the deuterium hydride molecule. Temperature programmed desorption spectra for both primary and secondary spillover materials, with the deuterium peak appearing first, have supported partitioning of the desorption behavior into reverse spillover and recombination directly from the atomic receptor. Desorption kinetics are faster relative to adsorption for hydrogen and deuterium on all carbon-based adsorbents in the study. Diffusion time constants are computed using a surface diffusion model based on Fick's second law. Kinetic data are studied for pressures from atmospheric to 100 atm and equilibrium adsorbed amounts to 5.5 mmol/g. Values follow an inverse relationship with amount and are 2 orders of magnitude higher (10(-4) s(-1)) than those reported for nitrogen physisorption on carbon adsorbents. Adsorption rates measured using volumetric techniques do not account for recombination, while desorption rates include both reverse spillover and recombination. Subtraction of the two rates yields the rate of recombination for hydrogen or deuterium atoms. The highest rate of recombination is observed for secondary spillover (1.3 x 10(12) atoms H/cm(2).s), and the maximum is nearly I order of magnitude higher than recombination for primary spillover. Recombination rates decrease with final adsorbed amount, supporting literature simulation of stable hydrogen clusters adsorbed to graphene sheets. The maximum deuterium atom recombination rate is greater than hydrogen, indicating that it is less stable on the receptor.